Axons and dendrites of neurons are long cellular extensions from neurons. The distal tip of an extending axon or neurite comprises a specialized region, known as the growth cone. Growth cones sense the local environment and guide axonal growth toward the neuron's target cell. Growth cones respond to several environmental cues, for example, surface adhesiveness, growth factors, neurotransmitters and electric fields. The guidance of growth at the cone involves various classes of adhesion molecules, intercellular signals, as well as factors that stimulate and inhibit growth cones. The growth cone of a growing neurite advances at various rates, but typically at the speed of one to two millimeters per day.
Growth cones are hand shaped, with broad flat expansion (microspikes or filopodia) that differentially adhere to surfaces in the embryo. The filopodia are continually active, some filopodia retract back into the growth cone, while others continue to elongate through the substratum. The elongations between different filopodia form lamellipodia.
The growth cone explores the area that is ahead of it and on either side with its lamellipodia and filopodia. When an elongation contacts a surface that is unfavorable to growth, it withdraws. When an elongation contacts a favorable growth surface, it continues to extend and guides the growth cone in that direction. The growth cone can be guided by small variations in surface properties of the substrata. When the growth cone reaches an appropriate target cell a synaptic connection is created.
Nerve cell function is greatly influenced by the contact between the neuron and other cells in its immediate environment (U. Rutishauser, T. M. Jessell, Physiol. Rev. 68:819 (1988)). These cells include specialized glial cells, oligodendrocytes in the central nervous system (CNS), and Schwann cells in the peripheral nervous system (PNS), which ensheathe the neuronal axon with myelin (an insulating structure of multi-layered membranes) (G. Lemke, in An Introduction to Molecular Neurobiology, Z. Hall, Ed. (Sinauer, Sunderland, Mass.), p. 281 (1992)).
While CNS neurons have the capacity to regenerate after injury, they are inhibited from doing so because of the presence of inhibitory proteins present in myelin and possibly also by other types of molecules normally found in their local environment (Brittis and Flanagan, Neuron 30:11-14 (2001); Jones et al., J. Neurosc. 22:2792-2803 (2002); Grimpe et al., J. Neurosci. 22:3144-3160 (2002)).
Several myelin inhibitory proteins that are found on oligodendrocytes have been characterized; e.g., NogoA (Chen et al., Nature 403:434-439 (2000); Grandpre et al., Nature 403:439-444 (2000)), myelin associated glycoprotein (MAG, McKerracher et al., Neuron 13:805-811 (1994); Mukhopadhyay et al., Neuron 13:757-767 (1994)) and oligodendrocyte glycoprotein (OM-gp, Mikol and Stefansson, J. Cell. Biol. 106:1273-1279 (1988)). Each of these proteins has been separately shown to be a ligand for the neuronal Nogo receptor-1 (Wang et al., Nature 417:941-944 (2002); Liu et al., Science 297:1190-93 (2002); Grandpre et al., Nature 403:439-444 (2000); Chen et al., Nature 403:434-439 (2000); Domeniconi et al., Neuron 35:283-90 (2002)).
Nogo receptor-1 is a GPI-anchored membrane protein that contains 8 leucine rich repeats (Fournier et al., Nature 409:341-346 (2001)). Upon interaction with an inhibitory protein (e.g., NogoA, MAG and OM-gp), the Nogo receptor-1 complex transduces signals that lead to growth cone collapse and inhibition of neurite outgrowth.
There is an urgent need for molecules that inhibit Nogo receptor-1 binding to its ligands and attenuate myelin-mediated growth cone collapse and inhibition of neurite outgrowth.